1. Field of the Invention
The present invention relates to a method of forming a semiconductor electrode on the surface of a semiconductor and a semiconductor device with electrodes formed by the method.
2. Description of the Related Art
The following technique has conventionally been adopted to form an ohmic contact between a metal electrode and a semiconductor.
An electrode material is deposited directly on a semiconductor, or an insulating film having a contact hole is formed on the semiconductor and the electrode material is deposited on the insulating film. An electrode forming process will be explained with reference to FIGS. 6A to 6C. First, an insulating film 10 such as oxide film is formed on a semiconductor substrate 20 and a portion of the insulating film 10 on which an electrode 13 is to be formed is eliminated by etching so that a contact hole 11 is formed. Secondly, an electrode material is deposited on the insulating film 10. A PVD (physical vapor deposition) apparatus such as sputtering apparatus or a CVD (chemical vapor deposition) is used for the deposition. Subsequently, a resist is applied to the overall electrode material, and the resist is exposed to light using a photomask to be developed so that a resist pattern is formed. The electrode material is processed by dry etching using the resist pattern so that an electrode 13 having a predetermined shape is formed at a position corresponding to a contact portion of the semiconductor substrate 20. Subsequently, part of a metal composing the electrode 13 is diffused into the contact portion of the semiconductor substrate 20. As the result of the diffusion, a mixed layer is formed in a boundary between the electrode 13 and the contact portion of the semiconductor substrate 20, whereby the electrode 13 and the contact portion of the semiconductor substrate 20 form an ohmic contact, thereby being formed into an ohmic electrode.
A large amount of impurities is previously introduced before the deposition of electrode material so that a high concentration impurity layer 12 of about 1020 cmxe2x88x923 is formed on the semiconductor substrate 20 on which an ohmic electrode is to be formed. Ion implantation, impurity deposition, etc. is employed for the impurity introduction and thereafter, a heat treatment is carried out so that the introduced impurities are activated or diffused.
In another method, a dopant metallic material which can serve as a donor or acceptor is deposited on the semiconductor substrate and thereafter, the dopant metallic material is sometimes diffused by heat treatment into the semiconductor substrate so that an N or P type layer.
A film former for forming an electrode forming material layer for the ohmic electrode on a semiconductor substrate includes PVD (physical vapor deposition) such as vapor deposition and sputtering, CVD (chemical vapor deposition). A document, xe2x80x9cULSI Technologyxe2x80x9d by C. Y. Chang, et al., MacGRAWHILL (1996), pp 379-395, describes such film formers in detail. A heat treatment such as sintering is thereafter applied to the semiconductor substrate on which the electrode forming material layer is formed. Consequently, the semiconductor material of the substrate is diffused into the electrode forming material layer to react therewith, whereupon an ohmic contact is formed.
In order that a desirable ohmic contact may be formed, a spontaneous oxide film present on the surface of the semiconductor needs to be removed. An etching step is required for removal of the spontaneous oxide film.
Furthermore, wire bonding in a semiconductor device is a primary example of an ohmic contact between a metal electrode and a metal, and both metals are connected together using an ultrasonic bonding which utilizes friction. In this case, the electrode metal serving as a backing is referred to as xe2x80x9cbonding pad,xe2x80x9d whereas the metal connected to the metal electrode is referred to as xe2x80x9cwire.xe2x80x9d
On the other hand, a method of directly bonding a metal to a semiconductor using the ultrasonic boding apparatus is scarcely used. The reason for this is that adhesiveness between the metal and a semiconductor is low and unpractical when the backing is the semiconductor.
As described above, the conventional method requires a number of steps in order that an ohmic electrode may be formed. More specifically, the conventional method requires an etching step removing a spontaneous oxide film on the semiconductor surface, a step for depositing an electrode material, a heat-treating step for sintering, an ion implanting step for forming an impurity layer in the semiconductor, etc. Accordingly, a period of time required for producing a semiconductor element is increased and hence increases the product cost.
Furthermore, an apparatus used in each step, for example, an ion implanting apparatus or a PVD apparatus is expensive equipment, thus increasing an equipment cost.
Additionally, a conventional wire bonding apparatus has often been used as means for connecting a metal wire and metal pad together in semiconductor chips. This apparatus connects the metal wire and metal pad together by frictional heat due to oscillation. However, in order that this may be applied to the connection of a semiconductor and a metal, the semiconductor surface needs to be provided with a region where a high concentration of impurities has been introduced. Even when the region is provided on the surface, it is difficult to ensure adhesiveness between a semiconductor and a metal, whereupon a desired ohmic characteristic cannot be achieved.
An object of the present invention is to provide a method of forming an ohmic connection between a semiconductor and a metal effectively in a short period of time without using expensive equipment which has been used in the conventional semiconductor preliminary treatment, the ohmic connection being difficult to be achieved by the wire bonding apparatus used in the conventional post-treatment, and a semiconductor device with electrodes formed by the method.
In order to achieve the object, the present invention provides a method of forming at least one electrode on a surface of a semiconductor, wherein a metal or alloy for the electrode is rubbed against a predetermined region of the semiconductor surface so as to be adhered by frictional force and frictional heat to the predetermined region of the semiconductor as an electrode and part of the adhered metal or a metal of the alloy is diffused into an inside of the semiconductor by the frictional heat thereby to be formed into an ohmic electrode substantially simultaneously when the metal or alloy is adhered by the frictional force and frictional heat to the predetermined region of the semiconductor.
In this case, the metal or alloy may be rotated so as to be brought into contact with the semiconductor surface as a technique for causing the frictional force and frictional heat. Furthermore, the metal or alloy for the electrode may be provided at least on an outer periphery of a rotator or may be formed into a brush shape.
Furthermore, the metal or alloy for the electrode may be provided at least on one side of a rotator.
Alternatively, the metal or alloy may be provided on an oscillator.
The metal for the electrode may become an n-type or p-type dopant.
In the present invention, the rotator or oscillator is operated at high speeds to be brought into contact with the semiconductor surface, whereby the metal or alloy for the electrode is adhered as an electrode to the semiconductor surface by the friction caused on a local part of the semiconductor surface, and local high frictional heat due to the high-speed operation results in effect of sintering. Consequently, an ohmic electrode can be formed.
Furthermore, when an impurity concentration on the semiconductor surface is increased so that an n+ layer and a p+ layer are formed, the n+ layer and the p+ layer can easily be formed on the semiconductor surface. Consequently, a high heat treatment such as sintering can be eliminated by the effect of frictional heat.